When radio signals are being transmitted between a transmitter and a receiver, various disturbance influences occur which must be taken into account in the signal detection process at the receiver end. First of all, the signal is subject to distortion which is caused by there generally being two or more possible signal propagation paths. Owing to reflection, scatter and diffraction of signal waves on obstructions, such as buildings, mountains and the like, the received field strength of the receiver is composed of a plurality of signal components which are generally of different strength and are subject to different delays. This phenomenon, which is known as multipath propagation, causes the distortion of the transmitted data signal that is known as intersymbol interference (ISI).
Other active subscribers represent a further cause of disturbances. The disturbances caused by these subscribers are referred to as multiple access interference (multi access interference: MAI). One frequently occurring scenario comprises the signal detection in the useful channel being seriously adversely affected by a dominant disturbance source or disturbance channel at that time.
First of all, only one channel will be considered, that is to say MAI will be ignored. This multipath transmission channel between the transmitter S and the receiver E can be modeled as a transmission filter H with a channel coefficient hk, as is illustrated in FIG. 1. The transmitter S feeds transmission symbols sk into the transmission channel, that is to say the channel model transmission filter H. A model adder SU allows an additive noise contribution nk to be taken into account, which is added to the transmission symbols Sk, filtered with hk, at the output of the channel model transmission filter H.
The index k denotes the discrete time in time units of the symbol clock. The transmission signals sk which are being filtered by the transmission filter H and on which noise is superimposed are received by the receiver E as the received signal xk, for which:
                              x          k                =                                            ∑                              i                =                0                            L                        ⁢                                          h                i                            ⁢                              s                                  k                  -                  i                                                              +                      n            k                                              (        1        )            where L represents the order of the transmission channel modeled by the filter H. As can be seen from equation (1), ISI is present since xk is dependent not only on sk but also on sk−1, . . . , sk−L.
FIG. 2 shows the channel model transmission filter H. The filter H has a shift register comprising L memory cells Z. Taps (a total of L+1) of them are in each case located in front of and behind each memory cell Z and lead to multipliers which multiply the values of the symbols sk, sk−1, . . . , sk−L which have been shifted into the shift register at the symbol clock rate T−1 via an input IN by the corresponding channel impulse responses h0, h1, . . . , hT. An output stage AD of the filter H adds the outputs of the L+1 multipliers. This thus results in an output signal OUT corresponding to equation 1.
The memory content of the channel model shift register describes the state of the channel. The memory content of the first memory cell on the input side contains the symbol sk−1 in the time unit k (which is multiplied by h1), while the further memory cells Z are occupied with the symbols sk−2, sk−3, . . . , sk−L. The state of the channel in the time unit k is thus defined unambiguously by the details of the memory contents, that is to say by the L-tuple (sk−L, sk−L+1, . . . , sk−1).
In the receiver E, the received signal values xk are known as sample values, and the channel impulse responses h0, h1, . . . , hL of the channel are estimated at regular time intervals. The object of the equalization process is to calculate the transmission symbols sk from this information. The following text is based on the use of a Viterbi equalizer for the equalization process.
Viterbi equalization is based on finding the shortest path through a state diagram of the channel, with this diagram being known as a trellis diagram. The channel states are plotted against the discrete time k in the trellis diagram. According to the Viterbi algorithm (VA), a branch metric, which represents a measure of the probability of the branch, is calculated for each possible branch between two states (previous state relating to the time unit k→destination state relating to the time unit k+1). The branch metrics are then added to the respective state metrics (which are frequently also referred to in the literature as path metrics) of the previous states (ADD). In the case of branches to the same destination state, the sums which are obtained in this way are compared (COMPARE). That branch to the destination state under consideration whose sum of the branch metric and state metric of the previous state is a minimum is selected (SELECT) and forms the extension of the path leading to this previous state to the destination state. These three basic VA operations are known as ACS-(ADD-COMPARE-SELECT-) operations.
While from the combination point of view, the number of paths through the trellis diagram increases exponentially as k rises (that is to say as time progresses), it remains constant for the VA. This is because of the selection step (SELECT). Only the selected path (“survivor”) survives, and can be continued. The other possible paths are rejected. Recursive path rejection is the core concept of the VA and is an essential precondition of using computation techniques to cope with the problem of searching for the shortest path (also referred to as the “best path”) through the trellis diagram.
The number of channel states (that is to say the number of occupancy options of the shift register H) in the trellis diagram, which is identical to the number of paths followed through the trellis diagram, is pL. In this case, p denotes the significance of the data symbols under consideration. The computation complexity of the VA accordingly increases exponentially with L. Since L should correspond to the length of the channel memory of the physical propagation channel, the complexity for processing the trellis diagram rises as the channel memory of the physical propagation channel increases.
One simple method to reduce the computation complexity is to base the trellis processing on a short channel memory L. However, this has a major adverse affect on the performance of the equalizer. A considerably more sensible measure to limit the computation complexity which does not seriously influence the quality of the equalizer is the decision feedback (DF) method. In the case of the DF method, the VA is based on a reduced trellis diagram, that is to say a trellis diagram in which only some of the pL channel states are taken into account, rather than all of them. When the trellis diagram is reduced to pLDF trellis states (LDF<L), the remaining L−LDF channel coefficients (which are not used for the definition of trellis states) are still taken into account by using them for the calculation of the branch metrics in the reduced trellis diagram.
A branch metric must be calculated both during the processing of the complete trellis diagram and during the processing of the reduced trellis diagram (the DF case) for each possible branch between two states. The branch metric is the Euclidean distance between the measured signal value or sample value Xk and a reconstructed “hypothetical” signal value which is calculated and “tested” in the receiver with respect to the destination state, the branch from the previous state to the destination state and the path history, taking into account the channel knowledge.
In order to explain this, let us assume by way of example that p=2 (binary data signal), that is to say there are 2L (DF case: 2LDF) trellis states (0, 0, . . . , 0), (1, 0, . . . , 0) to (1, 1, . . . , 1) comprising L tuples (DF: LDF tuples). Let us assume that one specific hypothetical previous state is defined by the shift register occupancy (aL, aL−1, . . . , a1) (in the DF case, only the LDF right-hand bits (aLDF, . . . , a1) of the shift register occupancy are used for the state definition). The hypothetically transmitted symbol (bit) 0 or 1 which leads from the previous state (aL, aL−1, . . . , a1) in the time unit k to the destination state (aL−1, aL−2, . . . , a0) in the time unit k+1 (DF: previous state (aLDF, . . . , a1) to the destination state (aLDF−1, . . . , a0)) is denoted a0. With or without DF, the branch metric BMk is:
                                                                        BM                k                            =                                                                                                            sample                      ⁢                                                                                          ⁢                      value                                        -                                          estimated                      ⁢                                                                                          ⁢                      signal                      ⁢                                                                                          ⁢                      value                                                                                        2                                                                                        =                                                                                                                                                                  X                          k                                                -                                                  (                                                                                                                    ∑                                                                  i                                  =                                  1                                                                L                                                            ⁢                                                                                                h                                  i                                                                ⁡                                                                  (                                                                      1                                    -                                                                          2                                      ·                                                                              a                                        i                                                                                                                                              )                                                                                                                      +                                                                                          h                                0                                                            ⁡                                                              (                                                                  1                                  -                                                                      2                                    ·                                                                          a                                      0                                                                                                                                      )                                                                                                              )                                                                                                            2                                    ⁢                                                                          ⁢                  for                  ⁢                                                                          ⁢                                      a                    i                                                  =                                  {                                      0                    ,                    1                                    }                                                                                        (        2        )            
The estimated signal value (also referred to in the following text as the estimated symbol) is a sum of products of a channel coefficient and a symbol. For the DF case, the term
      ∑          i      =      1        L    ⁢            h      i        ⁡          (              l        -                  2          ·                      a            i                              )      can also be split into a trellis contribution and a DF contribution:
                              BM          k                =                                                                        X                k                            -                              (                                                                                                    ∑                                                  i                          =                                                                                    L                              DF                                                        +                            1                                                                          L                                            ⁢                                                                        h                          i                                                ⁡                                                  (                                                      1                            -                                                          2                              ·                                                              a                                i                                                                                                              )                                                                                                            ︸                                              DF                        ⁢                                                                                                  ⁢                        contribution                                                                              +                                                                                    ∑                                                  i                          =                          1                                                                          L                          DF                                                                    ⁢                                                                        h                          i                                                ⁡                                                  (                                                      1                            -                                                          2                              ·                                                              a                                i                                                                                                              )                                                                                                            ︸                                              trellis                        ⁢                                                                                                  ⁢                        contribution                                                                              +                                                                                    h                        o                                            ⁡                                              (                                                  1                          -                                                      2                            ·                                                          a                              0                                                                                                      )                                                                                    ︸                                              hyp                        .                        symb                        .                        contribution                                                                                            )                                                          2                                    (        3        )            This means that the estimated symbol comprises two (in the DF case: three) contributions: a contribution which is defined by the hypothetically transmitted symbol a0 for the branch from the time unit k to the time unit k+1, the trellis contribution, which is given by the previous state with respect to the time unit k in the trellis diagram, and, in the DF case, the DF contribution is also added to this, because of the reduced trellis states.
With or without DF, the branch metric BMk is always the same. The computation saving in the case of VA with DF results, as already mentioned, from the smaller number 2LDF of trellis states to be taken into account in the processing of the trellis diagram, that is to say from the reduction in the trellis diagram.
If, furthermore, it is also intended to consider a disturbance channel (that is to say a second multipath transmission channel) for the equalization of a data signal, then joint VA equalization must be carried out on both channels (useful channel and disturbance channel). An overall trellis diagram which includes the states of both channels is set up for this purpose: one example: if p=2 (binary data signal) and L=4 for both channels, the trellis diagram for the useful channel has 16 states, and the trellis diagram for the disturbance channel likewise has 16 states. The “combinational” overall trellis diagram that is used as the basis for the joint VA equalization of both signals then comprises 16×16=256 states. If an additional DF bit is taken into account in each case (that is to say L=5, LDF=4), the overall trellis diagram still has 256 states, but two more DF bits (one each for each channel) are also added as the DF contribution in the calculation of the branch metrics.
The complexity for processing the overall trellis diagram is greater by a factor of 16 than the complexity for processing the trellis diagram for the useful channel on its own. When processing the trellis diagram under the control of a DSP (digital signal processor), a solution such as this leads to a very high MIPS load (MIPS: million instructions per second) on the DSP, so that no other applications can run on the DSP or can no longer run in an acceptable time. For a useful signal which is transmitted using the EDGE (enhanced data rates for GSM evolution) Standard (with p=8), equalization taking account of an interference source in the use of the overall trellis diagram is no longer possible in mobile radio practice, because of the excessively high DSP load.
If a further (that is to say a second) disturbance source is added, the overall trellis diagram already covers 16×16×16=4096 states (it is likewise assumed that p=2 and L=4 for the second disturbance source). In this case as well, the computation complexity for conventional VA equalization on the basis of an overall trellis diagram such as this can no longer be kept with.
It is already known from the document DE 103 23 407 A1 for a disturbance signal to be taken into account for equalization of a signal transmitted via a useful channel, in such a way that one trellis diagram for the disturbance channel and one trellis diagram for the useful channel are processed per time unit. The useful channel equalization is carried out using a DF method. In this case, the influence of the disturbance channel on the useful channel equalization is taken into account by a DF contribution, which is based on the best path determined during the equalization of the disturbance channel.
The procedure described in the document DE 103 23 407 A1 for useful channel equalization involving a disturbance channel will be explained with reference to FIG. 3. The illustration shows the processes P0 and P1 to be carried out in the time unit k. The disturbance channel is equalized for the time unit k in the process P0. A trellis diagram is used which includes exclusive states of the disturbance channel. The equalization can be carried out with or without DF. The disturbance channel equalization (the process p0) results in the best path of the disturbance channel for the time unit k. This best path of the disturbance channel can now be used as an additional DF contribution for the equalization of the useful channel (the process P1). The branch metric values BMUk(v(k)→v′(k+1)) for the useful channel are then calculated using the following equation:
                              BMU          k                =                                                                      x                k                            -                              (                                                                                                    ∑                                                  i                          =                          0                                                                          L                          I                                                                    ⁢                                                                        h                          Ii                                                ⁡                                                  (                                                      1                            -                                                          2                              ·                                                              a                                Ii                                                                                                              )                                                                                                            ︸                                              DF                        ⁢                                                                                                  ⁢                        contribution                        ⁢                                                                                                  ⁢                        disturbance                        ⁢                                                                                                  ⁢                        source                                                                              +                                                                                    ∑                                                  i                          =                                                                                    L                              DF                              U                                                        +                            1                                                                                                    L                          u                                                                    ⁢                                                                        h                          Ui                                                ⁡                                                  (                                                      1                            -                                                          2                              ·                                                              a                                Ui                                                                                                              )                                                                                                            ︸                                              DF                        -                                                  contribution                          ⁢                                                                                                          ⁢                          user                                                                                                      +                                                                                    ∑                                                  i                          =                          1                                                                          L                          DF                          U                                                                    ⁢                                                                        h                          Ui                                                ⁡                                                  (                                                      1                            -                                                          2                              ·                                                              a                                Ui                                                                                                              )                                                                                                            ︸                                              trellis                        ⁢                                                                                                  ⁢                        contribution                        ⁢                                                                                                  ⁢                        user                                                                              +                                                                                    h                        U0                                            ⁡                                              (                                                  1                          -                                                      2                            ·                                                          a                              U0                                                                                                      )                                                                                    ︸                                                                        hyp                          .                          symb                          .                          contribution                                                ⁢                                                                                                  ⁢                        user                                                                                            )                                      ⁢                          ❘              2                                                          (        4        )            In this case, hUi, i=0, 1, . . . , LU denote the channel coefficients for the useful channel, hIi, i=0, 1, . . . , LI, the channel coefficients for the disturbance channel, aUi the bits of the useful channel, aIi the bits of the best path of the disturbance channel, LU, the order of the model filter for the useful channel, LI the order of the model filter for the disturbance channel, and LDFU the number of trellis bits for the useful channel. In comparison to the equation (3), it is clearly evident that the branch metric values include a further DF contribution (“DF contribution disturbance source”), which results from the best disturbance source path determined in the process P0.
The alternating processes P0 and P1 are carried out in each time unit k. It should be noted that the best path in the disturbance channel (in the process P0) is in each case calculated for the same time unit k as the branch metric values in the useful channel (process P1). This ensures that the contribution of the current (time unit k) symbol in the disturbance channel is also taken into account for the equalization of the useful channel in the time unit k. The major difference in comparison to the scenario without any disturbance sources is thus the addition of the best disturbance channel path to the respective useful channel path, in the form of DF bits (that is to say bits which are used only in the calculation of the branch metric values and not for state definition in the useful channel trellis diagram). The processing of the disturbance channel trellis diagram (the process P0) is in contrast carried out without taking into account any DF contribution from the useful channel trellis processing.
FIG. 4 shows a further option for equalization of a useful channel in the presence of a disturbance source. In the case of the method shown in FIG. 4, which is described in the document DE 103 38 050 that was not published prior to this, the useful channel trellis diagram is processed twice and the disturbance channel trellis diagram is processed once per time unit k. Equalization of the useful channel (for the time unit k) is carried out at the start (the process P0). In this case, no DF contribution from another channel is taken into account. During the useful channel equalization that is carried out in the process P0, the best useful channel path is calculated for the time unit k (the best path for the time unit k, based on the normal definition, is that path which leads to the destination state (time unit k+1) which has the least state metric during processing of the trellis diagram). This best useful channel path as determined in the process P0 is now used as the “other channel DF contribution” for the DF equalization of the disturbance channel for the same time unit k. The influence of the useful channel is taken into account in this way in the equalization of the disturbance channel. The best path of the disturbance channel which was determined during the equalization of the disturbance channel (the process P1) is then used—still for the time unit k—for equalization of the useful channel once again (the process P2). This second equalization of the useful channel for the time unit k is significantly better than the first equalization carried out in the process P0, since it takes account of the influence of the disturbance channel. The quality of the useful channel equalization in the process P2 (FIG. 4) is also significantly better than the useful channel equalization in the process P1 shown in FIG. 3, since the best path determined for the time unit k in the disturbance channel is more reliable owing to the consideration of the disturbance channel by the useful channel in the process P1.
It should be noted that both the method illustrated in FIG. 3 and that illustrated in FIG. 4 result in considerable computation savings in comparison to the conventional method (processing of an overall trellis diagram which includes the combined disturbance source/user states). In the case of the processing illustrated in FIG. 3, only 2 (the number of the trellis diagram)×16 (the number of the states in a trellis diagram)=32 states need be taken into account per time unit. In the case of the processing illustrated in FIG. 4, the complexity is increased to 3 (the number of trellis diagrams)×16 (the number of the states in one trellis diagram)=48 states, which must be taken into account in the trellis processing operations per time unit. In both cases (FIGS. 3 and 4), considerably fewer states are thus considered—and therefore considerably fewer paths and state metrics are calculated—than in the case of the conventional method that has been explained (processing of the overall trellis diagram with 256 states).
The invention is based on the object of specifying a simple and powerful method for equalization of a signal transmitted via a useful channel, based on the DF method and taking into account at least one disturbance channel.